1. Field of the Invention
The present invention relates to an image forming method, and more particularly to an image forming method which is effective in obtaining a recorded image of a high quality.
2. Description of the Related Art
Recently, a full color laser beam printer and a full color thermal transfer printer are rapidly coming into widespread use as output terminals for a personal computer, a work station, and the like. In such a case, a required performance is to output data of characters, a line drawing, and an image. Namely, a multi-gradation output is requested.
As an apparatus of this kind, known is an apparatus disclosed in, for example, JP-A-9-1866.
The disclosed apparatus is basically a thermal transfer printer and relates to one kind of pseudo area gradation technique using the multi-level dither method in which gradation is expressed in each of unit blocks each configured by a plurality of dots. The input/output characteristics corresponding to the position of a pixel in each unit block which is divided by block dividing means are made different among each of the unit blocks, the density level of an image data is converted to that for recording a dot, and positions where dots of the same size are recorded are changed among the unit blocks to be dispersed, so that the regularities of the size and arrangement of recording dots are reduced as much as possible. According to this configuration, generation of moire can be reduced in number while not making the image structure visually conspicuous.
In the pseudo area gradation technique using the dither method including the conventional image forming method, the characteristics of the gradation conversion table are so monotonous that unit blocks of the same pattern are usually used for the same color. In the case where misregistration of even a small degree occurs in locating of a recording position of each color, therefore, color misalignment is produced. When the size of a unit block is increased, the degree of such color misalignment can be relatively reduced, and hence the misalignment can be made inconspicuous. However, this causes a problem in that the unit block itself becomes conspicuous. By contrast, when the size of a unit block is reduced, there arises a problem in that color misalignment is conspicuous.
It is an object of the invention to solve the problems. It is another object of the invention to provide an image forming method which, even when misregistration occurs in locating of a recording position of each color, can make color misalignment inconspicuous, and hence is effective in obtaining a recorded image of a high quality.